This invention relates to the production of metals and more in particular to an apparatus to electrolytically form a multivalent metal from a salt thereof.
Metals, such as titanium, have previously been produced from compounds thereof, for example, titanium tetrachloride, by electrolytic means as described in U.S. Pat. Nos. 2,789,943; 2,943,032; and 3,082,159. Generally, the titanium tetrachloride is introduced into a molten alkali or alkaline earth metal salt bath through appropriate means and electrolytically disassociated to plate metallic titanium on a cathode and to release elemental chlorine at an anode. Various means have been employed to separate the anode from the cathode in the titanium-bearing electrolytic cells.
A physical barrier, such as a diaphragm, positioned between the anode and cathode compartments is necessary to prevent an excessive flow of titanium ions from the cathode compartment into the anode compartment. If such an excessive ion flow occurs, titanium ions would be oxidized to titanium tetrachloride thereby reducing the cell efficiency. The diaphragm should also permit passage of chloride ions and a fused salt bath between the anode and the cathode compartments.
The diaphragm of U.S. Pat. No. 2,789,943 consisted of a perforate, electrically conductive metallic structure which, when in use, was interchangeably an anode or a cathode. The diaphragm was made a cathode to cause deposition of metallic titanium into the pores thereof and reduce the porosity of the diaphragm. The electrical polarity was reversed, making the diaphragm an anode to remove titanium therefrom, when the diaphragm became excessively impervious and reduced the electrolytic cell efficiency. Such a diaphragm of variable porosity is operable; however, it would be more desirable to have a diaphragm which would not necessitate constant monitoring and frequent metal plating thereon and etching therefrom.
Leone et al., Use of Composite Diaphragms in Electrowinning of Titanium, Bureau of Mines Report RI 7648 (1972) and Leone et al., High-Purity Titanium Electrowon from Titanium Tetrachloride, J. of Metals 18 (March 1967) describe porous, metal screen-ceramic composite diaphragms positioned between anodes and cathodes for use in the electrowinning of titanium. The metal screen-ceramic composite is more costly and has a lower strength than is desired for production operations.
The electrolytic cells of the prior art are operable; however, the barrier or diaphragm between the anode and cathode chambers has usually been deficient in strength characteristics needed for production-type electrolytic equipment or required continuous and careful regulation of the porosity during operation of the cell. An improved electrolytic cell for the electrowinning of metals using a diaphragm with adequate physical properties and a constant porosity, which need not be regulated during operation, is desired.